Building reverse flow what size opening between firebox and chamber?

A friend of mine (welder) wants to build a reverseflow smoker. We currently have 2 tanks, each about 20" (or 22") in diameter. ONe is around 5' long, the other 3'. Plan was to use the smaller one as the firebox. I think I've read that the firebox should be ~30% of the Smoking chamber in size. But I'm not sure what size the opening should be? OR does it really matter much?

Thank in advance.